what is a weed anyway?

Ralph Waldo Emerson says a weed is a "plant whose virtues have yet to be discovered." In weeding our garden, there's just a handful of unwanted plants that I keep recognizing. One is purslane, a succulent edible that you often see growing in sidewalk cracks. I've also seen it featured in salads in fancy restaurants. Here it is growing out the side of our raised bed:

...that's probably a $20 salad right there.

So I've been leaving it in the garden if it isn't crowding other plants too badly. I just learned from Wikipedia that:

As a companion plant, Purslane provides ground cover to create a humid microclimate for nearby plants, stabilizing ground moisture. Its deep roots bring up moisture and nutrients that those plants can use, and some, including corn, will "follow" purslane roots down through harder soil than they can penetrate on their own. It is known as a beneficial weed in places that don't already grow it as a crop in its own right.

Another common weed in our garden and around town is mallow (malva neglecta):
I'm not sure if it is a beneficial companion like purslane, but it is a demulcent, so I've been picking it to make into a mucilaginous tea, which was nice when I had a sore throat recently (it's also an ingredient in the Ricola cough drops I bought recently).